Interpretive Summary: Combinations of the insecticide pyrethrin combined with the insect growth regulator methoprene (Diacon II(R)) are used as aerosol insecticides to control the red flour beetle and the confused flour beetle in flour mills. However, there is little information on how long the residues from an aerosol application will give control of either of these two important pest species. We placed wheat flour or packaging materials inside a flour mill and exposed them to commercial aerosol formulations containing either 1% or 3% pyrethrin with the same amount of methoprene. Then, for each of two weeks for 16 weeks, we put larvae of the red flour beetle and confused flour beetle on the flour and packaging materials. Control was determined by whether or not the larvae could reach the adult stage. Both commercial formulations gave complete control of red flour beetles on all surfaces for 16 weeks, but on all surfaces the residues were less active against the confused flour beetle, and more of the larvae were able to reach the adult stage compared to the red flour beetle. Increasing the pyrethrin concentration from 1% to 3% did lead to a reduction in the number of confused flour beetle adults, indicating there may be an interaction between the pyrethrin and methoprene components of the formulation. The formulation with 1% pyrethrin will control the red four beetle, while the 3% pyrethrin formulation is needed to control the confused flour beetle.

Technical Abstract:
Wheat flour and different packaging surfaces (cardboard, flour bag, muslin bag, paper bag, pallet wrap, plastic overwrap, polyethylene) were exposed to aerosol formulations of either 1% active ingredient [AI] pyrethrin + methoprene or 3% [AI] pyrethrin + methoprene. Residual bioassays were conducted every two weeks for 16 weeks post-exposure to the aerosol by placing four-week-old larvae of the red flour beetle, Tribolium castaneum (Herbst), and the confused flour beetle, T. confusum (Jacqueline duVal), on treated flour or a treated packaging surface with untreated flour added. Mortality and morphological characteristics were assessed for a period of 30 days after adding larvae to the dishes. T. castaneum was clearly the more susceptible of the two species. Less than 2% of T. castaneum larvae exposed to aerosol treated flour or packaging surfaces emerged as normal adults, regardless of the pyrethrin concentration. Most of the T. castaneum larvae on treated flour did not advance to the pupal stage because they were either developmentally arrested or died as larvae. They were able to develop further on the treated packaging surfaces, but still could not emerge as adults. T. confusum larvae exposed to aerosol treated flour or packaging surfaces were able to develop to the pupal or adult stage. Emergence of normal-appearing adults from T. confusum larvae exposed on the packaging surfaces treated with 1% pyrethrin + methoprene gradually increased (range of 29.7 ± 2.9 to 49.0 ± 6.7%, depending on the surface), whereas adult emergence of larvae exposed to treated flour peaked at 10 weeks post-exposure. However, when T. confusum was exposed to 3% pyrethrin + methoprene treated flour or packaging surfaces, adult emergence was limited. Overall there were few significant differences attributable to the individual packaging surfaces. Pyrethrin has limited residual activity, so the decrease in adult emergence with the higher pyrethrin concentration may indicate an interaction with methoprene.